Spermatogenesis is a complex process of cell differentiation involving interactions between germ cells, Sertoli cells, and other somatic cells within the testis. One feature of this process is the appearance of several germ cell-specific constituents in a precise temporal sequence. Three areas of research have been pursued to further characterize these unique constituents, both in the acrosome and on the cell surface. (a) Monoclonal antibodies have been used to identify and characterize germ cell components expressed during restricted periods of spermatogenesis. Antibody ID4 reacts with multiple glycoconjugates that appear in the acrosome of early spermatids but are modified during the late haploid stages so that the determinant is no longer detected. In contrast, this antibody recognizes acrosomal glycoconjugates that are retained in guinea pig spermatozoa. Additional monoclonal antibodies that recognize cell surface constituents have been prepared against proteins excised from two-dimensional gels. (b) Since several glycosylated molecules have been identified on germ cell surfaces and in the acrosome, we have begun to analyze oligosaccharides in isolated spermatogenic cells. Unusual high- mannose oligosaccharides have been identified in both pachytene spermatocytes and round spermatids. Both cell types also have mannose 6-phosphate (M6P) receptors that mediate endocytosis of M6P-bearing ligands. (c) Interactions between Sertoli cells and germ cells at defined stages of spermatogenesis have been examined in short term cultures. When cultured in the presence of Sertoli cell-conditioned medium (SCM), pachytene spermatocytes and round spermatids maintain elevated viabilities and ATP levels. SCM contains multiple glycoproteins and its active fraction has stability characteristics that distinguish it from Sertoli cell growth factors described previously. Germ cell and Sertoli cell constituents, particularly those exhibiting stage and tissue specificity, are candidates for further studies exploring gene regulation and cell interactions during spermatogenesis.